The present invention relates to multi-conductor cables, and particularly to the structure of a miniature coaxial conductor pair for use in such cables.
Cables including numerous pairs of coaxial conductors for interconnecting elements of computer and other electronic systems must be small in size and highly flexible to enable their use in such systems. In order to keep the volume of such cable to a minimum, the coaxial conductor pairs inside the cable must accordingly be of extremely small diameter. Despite their small size, however, the coaxial conductor pairs must be highly resistant to breakage due to the flexing of the cable, while also having the ability to transmit signals rapidly with immunity from interference or leakage of signals between individual conductors.
The prevention of interference or leakage between such coaxial conductors during rapid signal transmission requires a high degree of electrical insulation between the conductors, which has led to the requirement of minimizing the dielectric constant of the insulation material to produce a coaxial conductor structure of the lowest possible capacitance per unit length. Minimizing of the capacitance between a pair of coaxial conductors maximizes the practical velocity of propagation of signals through the conductors, without which the speed of the overall system is inhibited and problems in synchronization among elements thereof may result.
It is known that insulating material of a low dielectric constant composed of polymeric fluorocarbon materials in various woven filament or wrapped tape configurations, such as those disclosed in Hawkins U.S. Pat. Nos. 4,332,976 and 4,440,973, and Perrault U.S. Pat. No. 4,340,773, enables signal propagation velocities of approximately 80% of the speed of light even though the thickness of the insulation is relatively small. In other known coaxial conductor pairs, the inner conductor is insulated from the outer conductor with a helical wrapping of tape composed of an expanded fibrous polytetrafluoroethylene (PTFE), which includes entrapped air in the material itself producing an even lower dielectric constant.
While the velocity of propagation achieved with such previously-available coaxial conductor pairs is excellent, the use of the insulating material in the form of woven filaments or tape in contact with the inner conductor of the coaxial pair has provided too little mechanical support for the inner conductor to prevent an unacceptably high rate of breakage of the inner conductor due to normal flexure of the cable. Accordingly many of these coaxial conductors have been susceptible to breakage of the conductors within short periods of time.
It is known to enclose an inner conductor of a coaxial conductor pair in an extruded solid insulation tubing of PTFE or other suitable material as shown, for example, in Perzel U.S. Pat. No. 2,636,923. Although such structure offers better mechanical support for the inner conductor, tubular solid PTFE or similar material does not have as low a dielectric constant as does a tape or filament structure of the same material, especially an expanded or fibrous tape or filament, and therefore a higher capacitance and lower velocity of signal propagation must be tolerated unless additional insulation is provided in some other way which adds diameter and complexity to the coaxial structure.
Accordingly, what is needed is a miniature coaxial conductor construction for use in a multi-conductor cable which withstands flexure for a significantly longer time than previous constructions without breakage of the inner conductor, has a high degree of flexibility despite its resistance to breakage, is extremely small in diameter to minimize the size of the multi-conductor cable in which it is incorporated and yet, despite its small size, has a low capacitance and corresponding high velocity of signal propagation.